Many processes involve the transport of large quantities of items in a conveying system. In some of these processes, it is essential that each individual item be separated from the items immediately in front of and/or behind it. For such cases, imperfections in the apparatus that replace the items in the conveying system make it necessary and useful to develop a system capable of detecting two or more overlapping items, commonly referred to as multiples.
Various methods for detecting multiples have been devised. For items of consistent thickness, mechanical thickness sensing (and for thinner, translucent material, detection and measurement of the attenuation of a radiation source passing through the material) have been employed. For mixed items of highly variable thickness other strategies are required. Typical approaches used have included comparison of item length to maximum item length, attempted separation by vacuum, and measurement of length or height before and after applying opposed forces to the two sides of the object or objects.
While existing detection methods for items of consistent thickness can be quite reliable, existing methods for the detection of multiples in a stream of highly variable items have shortcomings. Comparing item length to a maximum only detects those multiples which overlap in such a way as to exceed the maximum single piece length. The more variation in length among the items, the less this strategy will be successful. Using vacuum to pull at both sides of an item in an attempt to separate the item into two parts (if it is in fact a multiple) is limited in higher speed applications or applications where there is considerable variation in item stiffness or mass. This approach also tends to be bulky and noisy. The use of opposing forces on the sides of the item in order to change its apparent length or height (if it is in fact a multiple) has the drawback that for many classes of materials, the use of a force necessary to break the static frictional force or other force that tended to create the multiple in the first place will tend to damage a large proportion of items. It is also difficult to successfully implement such a system for a wide range of thicknesses.
Detection of unwanted overlapping objects is extremely important in automatic mail transporting systems, and also in systems in which an overlapping object may cause jamming or stoppage of the system. Automatic mail transporting systems are utilized for the efficient handling and routing of virtually millions of pieces of mail. With systems that require mail pieces to be separated and singulated as they move along a path, such as the United States Postal Services system, detection of overlapping mail pieces after exiting an upstream feeder is very important. While these upstream feeders output a very low percentage of overlapping documents (commonly referred to as "multiples"), many users of such systems require an even lower percentage of multiples.
In some of these systems, 25 to 35 thousand mail pieces per hour are fed into a transport, read (via optical character recognition (OCR) or bar code), and then sorted. The effect of an undetected multiple is generally to cause a piece to be sorted to the wrong destination. Given the huge volume of mail processed in the United States alone (177 billion in 1994), even a small percentage of undetected multiples results in a large cost for rehandling and significant number of pieces delayed in reaching their destination. These mail streams generally contain items with a very wide range of thickness, height, length, stiffness, color, interference (print) and mass. As a consequence (and given the limitations of existing detection technologies), most mail automation systems scan for doubles detectable by an item in excess of a maximum length, making no effort to detect other multiples, and resulting in a small but significant percentage of mis-sorted mail pieces.
The problem with present overlapping object detection systems is the inability to accurately detect overlapping objects that have been subjected to extensive handling or damage. The problem is that wrinkles and other small-scale distortions of the document surface can cause false edge indications. Since a large fraction of the mail processed by the U.S. Postal Service is typically sorted multiple times by hand and/or machine and is subject to damage during transport and processing, a practical device intended for use in processing this material must be able to discriminate between true edges and common surface deformations.
Accordingly, there is a need for an improved multiples detection technology for use in the handling of mail and other material which varies in mass, thickness and other physical characteristics. Further, there is a need for an improved overlapping object detector and new geometric dichotomous scanning technique capable of providing information on orientation, position and thickness of edges on the surface of an object that is accurate even when the surface is deformed or damaged. Such information can then be used to distinguish a true overlapping object from an object having a non-uniform surface. Additionally, there is a need for a low cost overlapping object detector of small-size to permit a plurality of such detectors to be installed within the object transport or feeder system and further, for a detector to scan both the upper and lower surfaces of the objects.